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%I #22 Jul 01 2022 18:03:34
%S 71,7777777777771,77777777777777777771,77777777777777777777771,
%T 7777777777777777777777777777771,
%U 7777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777777771
%N Primes of the form 70*R_k + 1, where R_k is the repunit (A002275) of length k.
%C Primes of the form (7*10^k - 61)/9. - _Vincenzo Librandi_, Nov 16 2010
%C The next term (a(7)) has 241 digits. - _Harvey P. Dale_, Jul 01 2022
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/77771.htm#prime">Prime numbers of the form 77...771</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = (70*10^A056688(n) - 61)/9 = (7*10^A099419(n) - 61)/9.
%t Select[Table[FromDigits[PadLeft[{1},n,7]],{n,100}],PrimeQ] (* _Harvey P. Dale_, Jul 01 2022 *)
%Y Cf. A002275, A056688 (corresponding k), A099419.
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Mar 27 2004
%E Edited by _Ray Chandler_, Mar 06 2012